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Ward unravels bacteria’s role in global nitrogen cycle

by Anne Sasso

July 13, 2006 4:12 p.m.

Ward, pictured here with partner Geoffrey Vallis, has spent a
considerable amount of time conducting research in Antarctica at Lake Bonney,
an ice-covered body of water with two lobes whose bacteria behave differently.
(photo by Andrew Gossen)

A rock props open the door to Bess Ward’s
office on the second floor of Guyot Hall in Princeton’s Department of
Geosciences. It’s a nod to the department’s acclaimed tradition of
geophysics, plate tectonics and “hard rock stuff,” as Ward put it,
although rocks play only a secondary role in her research.

Ward is a biogeochemist, which is a big-picture way of saying that
she does a little bit of everything — biology, geology and chemistry,
especially as they relate to the world’s oceans. She studies the nitrogen
cycle, the biological and chemical shuffle of nitrogen from soils to oceans to
atmosphere and back again.

Key to this movement is a host of microorganisms, particularly
bacteria or “bugs,” as Ward calls them, that play a starring role in the smooth
transition from one form of nitrogen to another. Understanding the bacteria’s
role in the nitrogen cycle is critical to unraveling the impact of human
activity on the planet’s delicate nitrogen balance.

In an attempt to better understand which bugs do what where, Ward
has collected them from waters all over the world, from Chesapeake Bay to the
Arabian Sea, the Gulf of Alaska to Antarctica. Beginning 14 years ago, she has
studied Lake Bonney, a double-lobed, permanently ice-covered lake in the Dry
Valleys of East Antarctica.

“This lake has some very weird chemistry going on in it, and we
think it’s the bacteria. There’s something keeping the bacteria from doing
their natural thing,” Ward explained.

The bacteria in question are denitrifiers, and their normal
function is to suck up all the nitrogen in the water and “make it go away” by
converting it to a benign form of nitrogen gas that constitutes 70 percent of
the atmosphere. But that’s not happening at Lake Bonney, at least not in the
east lobe of the lake. For the denitrifiers in the west lobe, however, it’s
business as usual. Ward is trying to figure out why the two lobes tell such
different stories.

The beauty of Lake Bonney is that it’s a contained system, a
laboratory of sorts, with an ice cap that keeps gas from escaping and two lobes
that look identical, but behave quite differently biochemically. “If we can
figure out what controls denitrification in this little system, maybe we’ll
learn something fundamental about the process,” Ward said.

While her research is basic science — “We’re not going to reverse
global warming or save the world” — the processes Ward studies are ubiquitous
and have economic importance. “There are a lot of practical applications for
the bugs,” Ward said.

Dangerously off kilter

As a natural byproduct of waste — from plants and animals, to
sewage and landfills — nitrogen takes on many forms. In the right forms, it
provides nourishment for plants (nitrogen fertilizers boost crop yields) and is
a building block of proteins, which ensure the smooth functioning of all cells.
But in the wrong forms, it can be trouble, creating smog and acid rain. For
eons, nature has maintained this delicate balance.

Certain bacteria are critical. Playing the role of mediator, they
maintain equilibrium among nitrogen’s different forms. In the soil and in most
bodies of water, nitrogen fixing bacteria take nitrogen from the atmosphere and
transform it into forms that nourish plants and animals. Plants and animals
generate another form of nitrogen, ammonium, which nitrifying bacteria
transform into nitrate. Finally, denitrifying bacteria complete the loop by
removing nitrate from ecosystems, turning it into gas and releasing it back to
the atmosphere.

These bacteria carry out the most important reactions that regulate
the supply and removal of nitrogen in biological systems. The problem is that
scientists fear that the bacteria aren’t keeping up.

Within the last century, humans have contributed unprecedented
amounts of nitrogen to the biosphere by burning fossil fuels, dumping untreated
sewage and using nitrogen fertilizers in large-scale agriculture. This has led
to higher concentrations of nitrous oxide, a nitrogen-based greenhouse gas, and
contributes to nitrogen-related algal blooms and fish kills in coastal
ecosystems. With such a large influx of man-made nitrogen, the Earth’s nitrogen
system is dangerously off kilter.

Fundamental to Ward’s research is developing an understanding of
the effects of pollution on the bacteria. By understanding how the diversity of
bugs in one body of water compares to that of another body and monitoring how
each reacts to pollution, she’s hoping to answer such questions as: Does
“healthier” water contain different bugs from polluted water? And can
scientists put that knowledge to use to restore balance?

“Bess’ questions are dynamic; the way she looks at systems is often
very fresh. She’s been advancing this field since her days as a graduate
student,” said Mary Voytek, an environmental microbiologist with the U.S.
Geological Survey, who is Ward’s former doctoral student and ongoing
collaborator. Ward is a leading researcher in her field, Voytek said. “She’s
interested in important scientific questions and completely open to pulling in
whatever technique or collaborator she needs to get at an answer,” she added.

From beaches to bugs

Ward didn’t set out to study bacteria. She became interested in
marine work mostly because it represented a departure from her surroundings
growing up in east central Alabama. “I think it was probably a form of escape,”
she said. “What could be further [from home] than the ocean?” Her interest in
science, however, comes naturally: Both her parents are chemists who taught at
Auburn University throughout Ward’s childhood.

She began her studies closer to the Great Lakes than an ocean — she
earned a bachelor’s degree in zoology from Michigan State University. In the
late 1970s and early 1980s, when Ward was working on her Ph.D. in biological
oceanography at the University of Washington in Seattle, it became clear to
researchers that bacteria were the ocean’s chemists. “That was a natural juxtaposition,”
Ward said, leading her into the study of how bacteria influence the ocean’s
nitrogen cycle.

For the past two decades, Ward’s career has moved largely in tandem
with that of her partner, Geoffrey Vallis, an oceanographer studying the
large-scale circulation of the atmosphere and ocean and the complex
interactions between the two. Keeping academic couples together is a challenge
facing higher education today, she said. “We’ve been really lucky.”

Both held positions at Scripps Institution of Oceanography at the
University of California-San Diego before moving to the University of
California-Santa Cruz in 1989, where Ward became chair of the ocean sciences
department in 1995. In 1998, they were both offered positions in Princeton:
Ward in the University’s Department of Geosciences and Vallis at the National
Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory,
which is affiliated with Princeton through the graduate program in atmospheric
and oceanic sciences.

How Ward came to reside in the geosciences department is largely a
testament to its “immense prescience” in the late ’90s in realizing that the
environmental geosciences, including oceanography, biogeochemistry and isotope
geochemistry, were going to play an increasing role in the future of the
geosciences, she said. This led the department to branch out by hiring
scientists who could help answer some of the exciting intellectual questions at
the interface of biology and geology.

“She’s definitely one of the most valuable members of our
department. We’re a very diverse department, and she’s the most
biologically-oriented member,” said Tony Dahlen, chair of the Department of
Geosciences. “We’re very pleased to have her here and certainly pleased to have
been able to attract her from Santa Cruz, where she had a real ocean next
door.”

Ward is now the William J. Sinclair Professor of Geosciences, a
professor of ecology and evolutionary biology and the director of Princeton’s
Program in Environmental Studies. She supervises two graduate students and a
cadre of postdoctoral researchers.

Voytek described her as an inspirational mentor who is not afraid
of new techniques and who encourages her students to explore all possibilities,
not limiting them to the current ways of thinking about a subject. “Anyone who
spends time with her hopes to pattern their own career after hers, ” Voytek
said. “She’s really set an incredible example.”

Ward also teaches a core course, “Environmental Science, Policy and
Management,” in the environmental sciences curriculum along with two advanced
undergraduate courses in environmental microbiology and biological oceanography
in the Department of Geosciences. She enjoys integrating aspects of her
research with teaching. “That always makes the subject come alive to the
students,” she said. She puts a lot of effort into her teaching, Dahlen said,
and it shows. “[Her classes] just get fabulous reviews,” he said. “She’s
definitely one of our best teachers.”

Back to the ice?

Funding for Ward’s Lake Bonney research is coming to an end, and
she returned in February from her final voyage to Antarctica. Has she
formulated any theories about the bacteria in Lake Bonney’s twin lobes? “I
think they’re just pickled,” she said.

The concentration of salt in the eastern lobe is high enough to be
toxic to most organisms. Ward’s bacteria aren’t quite dead, they can be
resuscitated ever-so-slightly under laboratory conditions, but they’re not
really alive either, she said. “It’s been a real mystery; it’s kept us intrigued
for a long time. I’m coming to the conclusion that it isn’t anything that’s
specific to this nitrogen process; it’s more a critical level of toxicity,” she
explained.

One mystery may be solved, but a new one revealed: How did the east
lobe get so salty? While the salt question may not be compelling enough to
entice her to travel to the end of the Earth again, one more especially
intriguing bug she’s found could just lure her back to Lake Bonney’s
wind-blasted shore. Eyes flashing, she said, “[To trace] a specific organism,
one that we know is important, in a weird system like that might be really
cool!”